"Today, we are excited to unlock this technology for high-performance games, by teaming up with Epic Games. By leveraging this new JavaScript optimization technology, Mozilla has been able to bring Epic's Unreal Engine 3 to the Web. With this port, developers will soon be able to explore limitless possibilities when it comes to porting their popular gaming titles to the Web."

The developer actually chimed in on stack overflow and talked about problems with getting it to run cross browser.

Since the Web Audio API is a non-standard Google invention so far, other browsers haven't implemented it. This means everywhere else you have to crowbar HTML5 audio in to something that might work for games. The way to do this is to recycle audio objects.

The player's laser in Space Blaster fires 10 times a second - and that's not including any other sound effects! As I mentioned earlier, Audio is kind of a heavyweight object, so if you're doing new Audio() 10+ times a second, lo and behold, the browser eventually dies and audio starts glitching up. However, you can drastically reduce the number of Audio objects created by recycling them.

So I am not really surprised this is the case. But you are completely right.